Plating buss and a method of use thereof

ABSTRACT

The present invention relates generally to a plating buss design and method for minimizing short circuit problems in PCB panel singulation. More particularly, the invention encompasses a serpentine plating buss which increases the PCB singulation process window thereby minimizing short circuit problems due to indexing errors caused by occasional manufacturing and equipment alignment problems. The serpentine plating buss design therefore increases board yield.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/492,650, filed Jul. 25, 2006, which is a continuation of U.S.application Ser. No. 10/861,847, filed Jun. 4, 2004, now U.S. Pat. No.7,181,837.

BACKGROUND OF THE INVENTION

The present invention relates generally to the field of printed circuitfabrication, and more particularly to an improved plating buss on asubstrate such as for example a printed circuit board (PCB), whichaddresses manufacturing variations and alignment problems associatedwith PCB singulation for improved board yield.

Basically, a printed circuit board consists of a sheet of rigidinsulating substrate such as phenolic, glass impregnated epoxy or thelike, having a pre-defined pattern of thin metallic—usually copper—foilconductive paths (so-called “traces”) appearing on one or both sides ofthe substrate. These traces collectively define all the electricalinterconnections among all the components and are routed betweenappropriate locations on the board.

Electrolytic plating is one method used to improve electricalconductivity and/or wire bondability in the trace termination areasprovided on a multi-layered printed circuit board (PCB). Generally, apanel populated with an array of PCBs is connected to one terminal ofeither a d.c. or a pulsed plating voltage source and placed in anelectrolyte in order to be plated. A metal to be deposited is thenconnected to the other terminal and similarly immersed in theelectrolyte. The transfer of the metal is accomplished via the ionscontained in the current flowing between the metal and the panel.

Since electroplating of metals requires that all sites on the panel tobe plated must be electrically connected to the plating bath, one priorart method electrically connects the traces to a common straight-lineplating buss (also known as a “tie” or “commoning” bar) for convenience.The straight-line plating buss is then used to provide the currentduring the plating process. However, a plating buss serves no usefulfunction after the electroplating process. Thus, for improved processefficiency, prior art methods have provided the straight-line platingbuss centered between adjacent PCBs defined on the panel, such thatduring a PCB singulation stage, the straight-line plating buss gets cutaway.

PCB singulation is the process of taking a finished panel, andseparating or depaneling the plurality of PCBs formed thereon intoindividual PCBs for a subsequent packaging process. One method of PCBsingulation is to use a dicing saw. The panel is mounted on a sawcarrier, mechanically, adhesively or otherwise, as known in the art. Thesaw carrier is then mounted on a stage of the dicing saw. Typically, thePCBs are arranged in rows and columns on the panel with the periphery ofeach PCB being rectangular. During the dicing process, the panel is sawnor diced with a rotating blade along a street lying between each of therows and columns thereof.

Once all cuts associated with mutually parallel streets having oneorientation are complete, either the blade is rotated 90° relative tothe panel or the panel is rotated 90°, and cuts are made through streetsin a direction perpendicular to the initial direction of cut. Since eachPCB on a conventional panel has the same size and rectangularconfiguration, each pass of the saw blade is incrementally indexed oneunit (a unit being equal to the distance from one street to the next) ina particular orientation of the panel. As such, the saw and the softwarecontrolling it are designed to provide uniform and precise indexing infixed increments across the surface of the panel.

As mentioned previously above, PCB singulation is also used to removeprocess remnants, such as straight-line plating busses used in the (postformation) plating process of the PCBs. An illustration of one prior artplating buss is shown in FIG. 1, wherein a PCB panel 100 has a pluralityof traces 102 formed thereon by electrodeposition, which are connectedby an associated straight-line plating buss 104. At the PCB singulationstage, a saw blade dices the panel 100 along a cut, which removes panelmaterial between parallel lines 106 and 108 in order to separateadjacent PCB segments 110. If the cut is properly aligned, parallellines 106 and 108 will flank a street 112 defined between the adjacentPCB segments 112 in order to also remove the straight-line plating buss104 which was formed there along. As can be imaged, dicing the panelalong the street 112 will thereby disconnect the associated traces 102.According the width on the straight-line plating buss 104 can beconsidered as the “process window” for the PCB singulation stage.

However, over time, the blade and/or stage of the dicing saw mayexperience drift due to indexing errors of its drive motor andassociated gearing. Additionally, variations in the PCB manufacturingprocess effecting PCB sizes and street locations, and variations inblade width due to uneven heating and wear can also result in indexingerrors. Such indexing errors result potentially in the location of cut,illustrated by parallel lines 106 and 108, moving off the street 112 andout of the process window. As illustrated by FIG. 2, moving off thestreet 112 creates the potential for shorted circuits due to the cutfailing to remove the straight-line plating buss 104 connecting thetraces 102 in one of the adjacent PCB segments 110. This circumstancedecreasing board yield by rejecting PCBs with shorted traces even thoughall other features on each PCB may be good.

SUMMARY OF THE INVENTION

In view of the foregoing considerations, the present invention isdirected to an improved plating buss, wherein the plating bussconnecting traces of adjacent printed circuit board (PCB) in a substratepanel are provided in a serpentine design. The serpentine plating busswidens the PCB singulation process window, thereby minimizing shortcircuit problems often seen in PCB singulation due to indexing errorsresulting from occasional manufacturing variations and alignmentproblems. Accordingly, the serpentine plating buss of the presentinvention increases board yield. Another benefit of the serpentineplating buss is that the sawn edge of each finished PCB may be improveddue to the fact that no longer must PCB singulation be directly on topof the entire plating buss in order to disconnect joined conductivetraces.

In one embodiment, a substrate panel containing a plurality of printedcircuit boards is disclosed. The substrate panel comprises a pluralityof conductive traces provided on the substrate, and a serpentine platingbuss interconnecting the plurality of conductive traces and providedbetween adjacent ones of the plurality of printed circuit boards.

In another embodiment, a method for manufacturing a printed circuit isdisclosed. The method comprises providing a substrate panel, providing aplurality of printed circuit boards on the substrate, the printedcircuit boards having a plurality of conductive traces provided on thesubstrate, and interconnecting the plurality of conductive traces with aserpentine plating buss provided on the substrate between adjacent onesof the plurality of printed circuit boards.

In another embodiment, a method for manufacturing a printed circuit isdisclosed. The method comprises providing a substrate having at least apair of adjacent segments, the substrate having a conductive serpentineplating buss interconnecting conductive traces of the pair of adjacentsegments, and singulating the substrate, the singulation removing aportion of the serpentine plating buss which disconnects the conductivetraces.

In still another embodiment, a method for manufacturing a printedcircuit board is disclosed comprising providing an unclad laminatedsubstrate panel used for forming a plurality of printed circuit boards,associating an image of a desired circuitry pattern with the panel, theimage comprises various circuitry traces interconnected by a serpentineplate buss, and developing the panel, wherein the desired circuitrypattern is defined on the panel.

These and other features and objects of the present invention will beapparent in light of the description of the invention embodied herein.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a top elevation of a portion of a substrate panel bearingconductive traces connected by a prior art straight-line plating buss,and illustrating the alignment of a singulation cut having no indexingerror which will result in no shorted traces;

FIG. 2 is a top elevation of a portion of a substrate panel bearingconductive traces connected by a prior art straight-line plating buss,and illustrating the alignment of a singulation cut having an indexingerror which will result in shorted traces;

FIGS. 3 a-3 d are side section views of a substrate panel showing astep-by-step explanation of a printed circuit manufacturing techniqueused to form the plating buss of the present invention;

FIG. 4 is a top elevation of a portion of a substrate panel bearingconductive traces connected by a plating buss according to the presentinvention, illustrating the alignment of a singulation cut having noindexing error which will result in no shorted traces;

FIG. 5 is a top elevation of a portion of a substrate panel bearingconductive traces connected by a plating buss according to the presentinvention, illustrating the alignment of a singulation cut having anindexing error which will result still in no shorted traces;

FIGS. 6 a-6 c are illustrations of various serpentine plating bussembodiments according to the present invention.

In the drawings, dimensions of materials have been modified for clarityof illustration and are not necessarily true to scale.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 3 a-3 d provides a step-by-step explanation of a printed circuitmanufacturing technique used to form a plating buss according to thepresent invention. In conjunction with this explanation, FIGS. 4 and 5each present a top elevation of a substrate panel having a plurality ofadjacent printed circuit boards (PCBs) thereon and bearing conductivetraces connected by a plating buss according to the present invention.FIGS. 6 a-6 c are illustrations of various serpentine plating bussembodiments according to the present invention.

With reference to FIG. 3 a, manufacturing of a plurality of PCBs 10begins similar to most other commonly used additive techniques. Namely,an unclad laminated substrate panel 12 used for forming the plurality ofPCBs 10 is first provided. The panel 12 is normally made of anonconductive material, such as phenolic, glass-impregnated epoxy,polymide, FR4, FR5 or the like. The panel 12 is then drilled inaccordance with a predetermined pattern, such as by a C-N-C drillingmachine. In particular, variously sized holes 14 are imparted throughthe panel 12 at a variety of locations, in the exact configuration of adesired hole pattern for each printed circuit. For ease of illustration,FIGS. 4 and 5 do not show any of the holes 14.

The panel 12 is then coated with an activating layer 16 which promotesthe adhesion of a conductive material, such as copper, to the uncladlaminant panel 12. Following coating, an outer surface of the activatinglayer 16 is coated with a resist material 18 as illustrated by FIG. 3 b.The resist material 18 is preferably a photopolymer plating resistsolution well-known in the art, and is normally light sensitive.

A photographic film image or artwork 20 of a desired circuitry pattern22 is then associated with the panel 12. In particular, the artwork 20provides a picture or image of various circuitry traces and is properlydesigned to selectively prevent light from passing through portions ofthe film. For example, with one well-known technique, the circuitrypattern is presented on the film in the form of an emulsion materialwhich prevents the passage of light. The remainder of the film, where nocircuitry is desired, is clear. A maskless, fully digital process, suchas for example, a Digital Micromirror Device (DMD) or Gradient LightValve (GLV) imaging system, may also be used to project a negative ofthe desired circuitry pattern 22, onto the panel 12 as is known in theart.

The desired circuitry pattern 22 includes a serpentine plating bussdesign according to the present invention, which connects together aplurality of traces. One embodiment of the serpentine plating buss 24 isillustrated by FIGS. 4 and 5, which connects a plurality of traces 26 inan “accordion” fashion. It is to be appreciated that the plating bussdesign of the present invention need not be continuous, equally sized,accordion shaped, or rectangular in dimension. For example, asillustrated by FIGS. 6 a-c, the serpentine plating buss of the presentinvention may be a saw toothed pattern 40, a triangular pattern 42, acurvilinear pattern 44, or combinations thereof. A discussion of theadvantages of having such a serpentine plating buss according to thepresent invention is provided in a later section.

Referring back to FIG. 3 b, with the artwork 20 in place, the panel 12is then “exposed” to ultraviolet light 28. As previously described, theartwork 20 is designed to selectively allow and/or prevent passage ofthe ultraviolet light 28 at desired locations. The resist material 18 isnormally configured to “cure,” harden or otherwise react in response toexposure to ultraviolet light such that it is impervious to developerchemistry. At locations on the panel 12 where ultraviolet light isprevented from reaching the resist material 18 (i.e., the desiredcircuitry pattern), the resist material 18 will not cure, such that itwill be attacked by developer chemistry.

The panel 12 is then “developed”. With this commonly-used technique, anyresist material 18 not cured during exposure is removed from the panel12. As illustrated by FIG. 3 c, only cured portions of the resistmaterial 18 remain on the panel 12. Following developing, the desiredcircuitry pattern 22 having the design of the serpentine plating buss 24is defined on the panel 12. In particular and at this stage, thecircuitry pattern 22 is defined by the activating layer 16 not otherwisecovered by the resist material 18.

The panel 12 is then processed through an energized plating bath 30 todeposit an electrolytic material layer, such as metals or preciousmetals like copper, silver, gold, platinum, nickel, tin, and the likes,onto the desired circuitry pattern 22. During the electroplatingprocess, the resist material 18 resists or shields the electrolyticmaterial from plating to certain areas of the panel 12, wherein theelectroplated material is deposited only on exposed portions of theactivating layer 16. In this manner, only the desired circuitry pattern22 receives the electroplated material. The panel 12 is then passed toadditional (post plating buss formation) electroplating processes,wherein the formed plating busses 24 are then used to make electricalcontact for improved electrical conductivity and/or wire bondability intermination (e.g. “trace”) areas of each of the printed circuit board 10provided on panel 12.

After plating, panel 12 is then subjected to a “stripping” process.During the stripping process, the resist material 18 is removed, leavingthe plated material layer 32 and other optional electroplated materiallayers (not shown) as illustrated by FIG. 3 d. The plated material layer32 is provided on the panel 12 in the desired circuitry pattern 22 whichincludes the serpentine plating buss, such as for example, the accordionplating buss 24 illustrated by FIGS. 3 and 4, or one of the platingbusses illustrated by FIGS. 6 a-6 c. After stripping, the panel 12 isthen singulated into individual PCBs 10.

With refer to FIG. 3, a conventional saw blade is employed at the PCBsingulation stage. If there is no indexing errors, the saw blade willremove the material of the panel located between parallel lines 34 and36 as a cut is made along a street 38. As can be imaged, making such acut will separate the adjacent boards 10 and remove a portion of theserpentine plating buss 24, which disconnects the associated traces 26.Since the design of the serpentine plating buss 24 zigzags over thestreet 38, the entire plating buss need not be removed in order todisconnect the traces, unlike the straight-line plating buss shown inFIGS. 1 and 2.

It is also to be appreciated that the “process window” defined by theserpentine plating buss 24 is larger than the prior art straight-lineplating buss shown in FIGS. 1 and 2. In this manner, even if over time,indexing errors result in the cut moving from the center of the street38, the saw blade will still remove a portion of the serpentine platingbuss 24 and disconnect the traces 26 in both adjacent boards 10, such asillustrated by FIG. 4. Accordingly, board yield is increased as fewerPCBs are rejected with shorted traces. In one embodiment, a panel 12provided with the serpentine plating buss of the present inventionenlarges the PCB singulation process window to greater or equal to about±0.165 mm from centerline of the street, as compared to a prior artprocess windows of less than or equal to about ±0.085 mm from centerlineof the street. It is to be appreciated that the process window of thepresent invention is solely dependent upon the width of the serpentinepattern, whereas the prior art process window is dependent on saw bladewidth, buss line width, positional accuracy of the saw blade, andpositional accuracy of the buss to the fixing feature.

Although the additive method was described in the formation of theplating buss of the present invention, those skilled in the artrecognize that the subtractive method of forming a PCB may also be used.For example, in the subtractive method, a plurality of boards defined bya substrate is provided having at least one surface coated with aconductive material, such as a metal like copper. The circuitry patternincluding an embodiment of a plating buss design according to thepresent invention is then printed onto the conducive material-coatedsurface of the board by a resist material. The remaining exposedconductive material-coated surface is etched away, and the resist isthen removed leaving conductive circuitry having the serpentine platingbuss of the present invention interconnecting conductive traces andadjacent PCB segments.

Additionally, although dicing was described as a suitable method for PCBsingulation, the plating buss according to the present is alsobeneficial to panel subjected to other PCB singulation methods, such asfor example, punching or stamping.

Thus, while certain representative embodiments and details have beenshown for purposes of illustrating the invention, it will be apparent tothose skilled in the art that various changes in the invention disclosedherein may be made without departing from the scope of the invention,which is defined in the appended claims.

1. A substrate panel comprising: a plurality of printed circuit boardssuitable for singulation using a saw blade having a width defining afirst process window, each printed circuit board having a plurality ofconductive traces provided on a planar surface of the substrate; and aserpentine plating buss provided on said planar surface between adjacentones of said plurality of printed circuit boards and interconnectingsaid plurality of conductive traces, wherein said serpentine platingbuss extends laterally in a full pattern a distance greater than thewidth of the saw blade such that said serpentine plating bus defines asecond process window significantly larger than the first processwindow.
 2. The substrate panel of claim 1 wherein the serpentine platingbuss is provided in an accordion pattern.
 3. The substrate panel ofclaim 1 wherein the serpentine plating buss is continuous from one endof the substrate panel to another end of the substrate panel.
 4. Thesubstrate panel of claim 1 wherein the serpentine plating buss isnon-continuous from one end of the substrate panel to another end of thesubstrate panel.
 5. The substrate panel of claim 1 wherein theserpentine plating buss is provided in a saw toothed pattern.
 6. Thesubstrate panel of claim 1 wherein the serpentine plating buss isprovided in a triangular pattern.
 7. The substrate panel of claim 1wherein the serpentine plating buss is provided in a curvilinearpattern.
 8. The substrate panel of claim 1 wherein the serpentineplating buss is provided in a pattern selected from the group comprisingaccordion, rectangular, saw toothed, triangular, curvilinear pattern,and combinations thereof.
 9. The substrate panel of claim 1 wherein thesubstrate panel further comprises a plurality of various sized holes.10. A substrate panel comprising: a plurality of printed circuit boardssuitable for singulation using a saw blade having a width defining afirst process window, each printed circuit board having a plurality ofconductive traces provided on a planar surface of the substrate; aserpentine plating buss provided on said planar surface between adjacentones of said plurality of printed circuit boards and interconnectingsaid plurality of conductive traces, wherein said serpentine platingbuss extends laterally in a full pattern a distance greater than thewidth of the saw blade such that said serpentine plating bus defines asecond process window significantly larger than the first processwindow; and said serpentine plating buss is provided in a patternselected from the group comprising accordion, rectangular, saw toothed,triangular, curvilinear pattern, and combinations thereof.